A(1-7) stimulates hematopoietic recovery, including thrombocytopenia, and reduces mucosal lesions after intravenous chemotherapy. Stimulation of hematopoietic recovery including reduction of thrombocytopenia through increased progenitor proliferation after treatment with A(1-7) occurs after both total-body irradiation (TBI) and intravenous chemotherapy. In this application, we will employ a sequential approach to define the optimal dose-schedule in mice after irradiation, determine the window of time following radiation when A(1-7) is effective in reducing RIT, assess the biological function of platelets that enter the peripheral blood after A(1-7) therapy and further characterize the mechanisms of action whereby A(1-7) reduces RIT. The objectives for these studies and experimental approaches are listed below: Objective 1: We will first establish the effective dose range of A(1-7) required to stimulate reconstitution of platelet concentrations after ionizing radiation. This will be measured by establishing the dose reduction factor for the amount of radiation required to establish a nadir of 60,000 platelets/[unreadable]l over a range of doses of A(1-7) with measurements at days 7, 14, 21 and 30 after radiation. Objective 2: Using the total weekly dose determined in Objective 1, we will determine if divided daily dosing, alternate day dosing, or twice per week dosing provides the optimal response of platelet recovery following radiation exposure. Objective 3: We will determine the maximum time interval between irradiation and A(1-7) treatment (using the optimal dose schedule determined in Objective 2 to optimize platelet recovery from RIT. All drugs share an optimal therapeutic window encompassing frequency of dosage and maximum allowable interval between radiation exposure and drug intervention. Objective 4: We will test the biological function of platelets that enter the peripheral circulation in response to A(1-7) therapy following radiation exposure. Objective 5: We hypothesize that the A(1-7) mediated recovery from RIT occurs through modification of megakaryocytic lineages in the bone marrow through activation of the MAS receptor. Studies will be conducted to understand the mechanism by which A(1-7) modifies platelet recovery after TBI. The first set of studies will establish alterations in angiotensin receptors expression by TBI with and without treatment with A(1-7). Further, the effect of A(1-7) administration on the number of megakaryocyte progenitors and megakaryocytes (number and ploidy) after in intact and angiotensin receptor (including MAS) knock out mice will be determined. This application will develop a product aimed at the mitigation of post-radiation thrombocytopenia. With the threat of accidental or deliberate exposure of large populations to ionizing radiation, mitigation of side effects of such exposure, such as thrombocytopenia, that cause high degrees of morbidity and mortality is of importance.